An aeroplane tire needs more particularly to meet requirements of wear resistance and endurance. Endurance means the ability of the tire to resist over time the cyclic stresses to which it is subjected. When the tread of a tire is worn, which marks the end of a first useful life, the tire is retreaded, namely the worn tread is replaced with a new tread to allow a second service life. An improved resistance to wear means that a higher number of landings can be made per service life. Improved endurance means that the number of service lives that the same tire can have is increased.
In general, a tire comprises a tread intended to come into contact with the ground via a tread surface and connected by two side walls to two beads which are intended to provide a mechanical connection between the tire and the rim on which it is mounted.
In what follows, the circumferential, axial and radial directions respectively refer to a direction tangential to the tread surface of the tire in the direction of rotation of the tire, to a direction parallel to the axis of rotation of the tire and to a direction perpendicular to the axis of rotation of the tire. “Radially on the inside or respectively radially on the outside” means “closer to, respectively further away from, the axis of rotation of the tire”. “Axially on the inside or respectively axially on the outside” means “closer to or respectively further away from the equatorial plane of the tire”, the equatorial plane of the tire being the plane passing through the middle of the tread surface and perpendicular to the axis of rotation of the tire.
A radial tire more particularly comprises a reinforcement comprising a crown reinforcement, radially on the inside of the tread, and a carcass reinforcement, radially on the inside of the crown reinforcement.
The carcass reinforcement of an aeroplane tire generally comprises a plurality of carcass layers extending between the two beads and distributed between a first family and a second family.
The first family consists of carcass layers wrapped, within each bead, from the inside towards the outside of the tire, around a circumferential reinforcing element referred to as a bead wire to form a turn-up the end of which is generally radially on the outside of the radially outermost point of the bead wire. The turn-up is that portion of the carcass layer that lies between the radially innermost point of the carcass layer and the end thereof. The carcass layers of the first family are the carcass layers closest to the interior cavity of the tire and therefore the axially innermost ones in the side walls.
The second family consists of carcass layers extending, within each bead, from the outside towards the inside of the tire as far as an end which is generally radially on the inside of the radially outermost point of the bead wire. The carcass layers of the second family are the carcass layers closest to the exterior surface of the tire and therefore axially outermost in the side walls.
Usually, the carcass layers of the second family are positioned, along their entire length, on the outside of the carcass layers of the first family, i.e. they, in particular, envelop the turn-ups of the carcass layers of the first family.
Each carcass layer of the first and second family is made up of mutually parallel reinforcing elements making an angle comprised between 80° and 100° with the circumferential direction.
The reinforcing elements of the carcass layers are usually cords made up of spun textile filaments, preferably made of aliphatic polyamide or of aromatic polyamide and characterized by their mechanical extension properties.
The mechanical extension properties of the textile reinforcing elements, such as the modulus of elasticity, the elongation at break and the strength at break, are measured after initial conditioning. “Initial conditioning” means that the textile reinforcing elements are stored for at least 24 hours, prior to measurement, in a standard atmosphere in accordance with European Standard DIN EN 20139 (temperature of 20±2° C.; relative humidity of 65±2%). The measurements are then taken in the known way using a ZWICK GmbH & Co (Germany) tensile testing machine of type 1435 or type 1445. The textile reinforcing elements undergo tension over an initial length of 400 mm at a nominal rate of 200 mm/min. All the results are averaged over 10 measurements.
In use, an aeroplane tire is subjected to a combination of load and pressure which leads to a high degree of flexing, typically in excess of 30%. The degree of flexing of a tire is, by definition, its radial deformation, or its radial height variation when the tire changes from an unladen inflated state to a statically laden inflated state under pressure and load conditions as defined for example by the Tire and Rim Association or TRA standard. It is defined by the ratio of the radial height variation of the tire to half the difference between the outside diameter of the tire, measured statically in an unladen state inflated to the reference pressure, and the maximum diameter of the rim, measured on the rim flange. The TRA standard particularly describes the squashing of an aeroplane tire in terms of its squashed radius, i.e. the distance between the axis of the wheel of the tire and the plane of the ground with which the ground is in contact under the reference pressure and loading conditions.
An aircraft tire is also subjected to a high inflation pressure, typically in excess of 9 bar. This high pressure level entails a high number of carcass layers, because the carcass reinforcement is dimensioned to provide the tire with the ability to resist this level of pressure with a high factor of safety. By way of example, the carcass reinforcement of a tire with a service pressure, as recommended by the TRA standard, of 15 bar needs to be rated to withstand a pressure of 60 bar, assuming a factor of safety of 4. With the textile materials currently employed for the reinforcing elements, such as aliphatic polyamides or aromatic polyamides, the carcass reinforcement may, for example, comprise at least 5 carcass layers.
In use, the mechanical stresses of running induce flexing cycles in the beads of the tire which wrap over the rim flanges. These flexing or bending cycles generate in particular, within the carcass layer portions situated in the region of bending over the rim, variations in curvature which combine with the variations in elongation of the reinforcing elements of the carcass layers. These variations in elongation or deformations, particularly in the axially outermost carcass layers, may have negative minimum values corresponding to their being in compression. This compressive loading is likely to cause fatigue failure of the reinforcing elements and therefore premature tire degradation.
Those skilled in the art furthermore know that carcass layers comprising reinforcing elements made of aromatic polyamides have a low compression strength and are particularly sensitive to fatigue failure in compression.
In order to improve the fatigue strength of the beads of a tire intended to carry heavy loads and inflated to a high pressure, such as an aeroplane tire, document EP 0 567 521 describes a carcass reinforcement in which the ends of the axially outermost carcass layers are positioned between the respective turn-ups of the axially innermost carcass layers and are wrapped from the inside towards the outside of the tire around a bead wire.
The carcass reinforcement, as described in document EP 1 381 525, comprises at least two carcass layers of a first family, wrapped from the inside towards the outside of the tire around a bead wire, and at least one carcass layer axially on the outside of the carcass layers of the first family and their respective turn-ups. The solution proposed for improving the endurance of the beads of an aeroplane tire lies in the replacement of the carcass layers formed of reinforcing elements made of aliphatic polyamide with carcass layers formed of hybrid reinforcing elements, namely elements formed of spun filaments of different moduluses. The reinforcing elements of the carcass layers are cords formed by twisting and folding at least one spun yarn that has an elastic modulus in extension of at least 2000 cN/tex, with an overtwisted or not overtwisted spun yarn with a modulus of elasticity in extension of at most 1500 cN/tex, said moduluses of elasticity in extension of said spun yarns being measured for a tensile force equal to 0.1 times the breaking strength of a spun yarn.